Stabilizing agent for profile control gels and polymeric gels of improved stability

ABSTRACT

An aqueous polymeric gel-forming composition of improved stability for selectively plugging highly permeable zones in subterranean oil-bearing formations. The composition comprises an aqueous solution of a water-dispersable polymer present in a viscosifying amount, a crosslinking agent present in an amount sufficient to cause gelation and a stabilizing agent in an amount effective to reduce syneresis of the gelled composition.

FIELD OF THE INVENTION

This invention relates to a novel crosslinked gel-forming composition ofimproved stability which is useful in the control of permeability insubsterranean oil-bearing formations during enhanced oil recoveryoperations and, more particularly, to the use of an additive to form agel of improved stability. Use of such gel can yield improved sweepefficiency during fluid flooding operations.

BACKGROUND OF THE INVENTION

In the production of oil from subterranean formations, it is usuallypossible to recover only a small fraction of the total oil present inthe formation by so-called primary recovery methods which utilize onlythe natural forces present in the reservoir. To recover oil beyond thatwhich is produced by primary methods, a variety of supplementalproduction techniques have been employed. In these supplementaltechniques, commonly referred to as secondary recovery operations, afluid is introduced into the oil-bearing formation in order to displaceoil to a production system comprising one or more production wells. Thedisplacing or "drive" fluid may be an aqueous liquid such as brine orfresh water, a gas such as carbon dioxide, steam or dense-phase carbondioxide, an oil-miscible liquid such as butane, or an oil andwater-miscible liquid such as an alcohol. Often, the most cost-effectiveand desirable secondary recovery methods involve the injection of anaqueous or carbon dioxide flooding medium into an oil-bearing formation,either alone or in combination with other fluids. In practice, a numberof injection and production wells will be used in a given field arrangedin conventional patterns such as a line drive, a five spot or invertedfive spot, or a seven spot or inverted seven spot.

In the use of the various flooding techniques, it has become a commonexpedient to add various polymeric thickening agents to the drive fluidto increase its viscosity to a point where it approaches that of the oilwhich is desired to be displaced, thus improving the displacement of oilfrom the formation. The polymers used for this purpose are often said tobe used for "mobility" control.

Another problem encountered is that certain injected drive fluids may bemuch lighter than the reservoir fluids and thus separate by gravity,rising toward the top of the flowing region and resulting in thebypassing of the lower regions. This phenomena is known as gravityoverride.

Also encountered in the use of various flooding techniques is asituation caused by the fact that different regions or strata often havedifferent permeabilities. When this situation is encountered, the drivefluid may preferentially enter regions of higher permeability due totheir lower resistance to flow rather than the regions of lowpermeability where significant volumes of oil often reside.

It therefore is often desirable to plug the regions of highpermeability, or "thief" zones, either partly or entirely, so as todivert the drive fluid into regions of lower permeability. Themechanical isolation of these thief zones has been tried but verticalcommunication among reservoir strata often renders this methodineffective. Physical plugging of the high permeability regions bycements and solid slurries has also been tried with varying degrees ofsuccess; however, these techniques have the drawback thatstill-productive sites may be permanently closed.

As a result of these earlier efforts, the desireability of designing aviscous slurry capable of sealing off the most permeable layers so thatthe drive fluid would be diverted to the underswept, "tighter" regionsof the reservoir, became evident. This led to the use of oil/wateremulsions, as well as gels and polymers for controlling the permeabilityof the formations. This process is frequently referred to as "profile"control, a reference to the control of the vertical permeability profileof the reservoir. Profile control agents which have been proposedinclude oil/water emulsions, gels, e.g., lignosulfate gels and polymericgels, with polymeric gels being the most extensively applied in recentyears.

There are a variety of materials commercially available for profilecontrol, all of which perform differently and have their own, oftenunique, limitations. Among the many polymers examined thus far arepolyacrylamides, polysaccharides, celluloses, furfural-alcohol andacrylic-epoxy resins, silicates and polyisocyanurates. A major part ofthe work conducted in this area has dealt with polyacrylamides.Polyacrylamides have been used both in their normal, non-crosslinkedform as well as in the form of crosslinked metal complexes, asdescribed, for example, in U.S. Pat. Nos. 4,009,755, 4,069,869 and4,413,680. Shear degradation during injection and sensitivity toreservoir brines tend to diminish the beneficial effects derived fromthese polyacrylamides.

Proposals have been made for the use of inorganic polymers, especiallyinorganic silicates, as profile control agents. For example, U.S. Pat.Nos. 4,009,755 and 4,069,869 disclose the use of inorganic silicates forthis purpose. In the profile control method described in these patents,an organic polymeric profile control agent such as a crosslinkedpolyacrylamide or polysaccharide is first injected into the reservoir,followed by an aqueous solution of an alkaline metal silicate and amaterial that reacts with the silicate to form a silicate gel whichplugs the high permeability regions in the formation. An alkaline metalsilicate is typically used as the source of silica and the gelling agentis usually an acid or acid-forming compound such as a water solubleammonium salt, a lower aldehyde, an aluminum salt or an alkaline metalaluminate.

The problem, however, with many inorganic silicates is that theirsolutions are often quite viscous and stable only under alkalineconditions. As soon as conditions become acidic, a silicate gel isformed. Although this is the desired reaction for plugging theformation, it may occur prematurely. For example, gelation may beginbefore the solution has had an adequate opportunity to enter the highpermeability regions of the formation, cutting off the possibilities forfurther injection of plugging material.

Other attempts have been made to achieve profile control. One suchattempt is described in U.S. Pat. No. 4,498,539 to Bruning, whichdiscloses delayed gelable compositions for injection of a waterthickening amount of a polymer capable of gelling in the presence of acrosslinking agent so that after the composition has penetrated into anunderground formation and positioned itself preferentially in the highlypermeable strata, the delayed gelation is triggered by in-situhydrolysis of an ester which reduces the pH of the composition to thegelable range thereby producing in-depth plugging of the strata with thegelled polymer.

A group of polymeric thickeners which has received considerableattention for use in waterflooding is xanthan polysaccharides. Xanthanpolysaccharides are produced by the action of bacteria of the genusXanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and3,383,307 disclose mobility control by the use of polysaccharides in thepresence of polyvalent metal ion crosslinking agents. U.S. Pat. No.3,810,882 discloses the possibility of using certain reducible complexmetal ions as cross-linking agents for polysaccharides. U.S. Pat. Nos.4,078,607 and 4,104,193 describe a method for improving the efficiencyof waterflooding operations by a particular polysaccharide prehydrationtechnique. U.S. Pat. No. 4,413,680 describes the use of crosslinkedpolysaccharides for selective permeability control in oil reservoirs.

U.S. Pat. No. 3,908,760 describes a polymer water-flooding process inwhich a gelled, water-soluble Xanthomonas polysaccharide is injectedinto a stratified reservoir to form a slug, band or front of gelextending vertically across both high permeability and low permeabilitystrata. This patent also suggests the use of complexed polysaccharidesto block natural or man-made fractures in formations. The use ofpolyvalent metal ions for crosslinking xanthan polysaccharides and otherpolymers which are to be used for permeability control is described inU.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680. The use ofphenol/aldehyde crosslinking agents with xanthan polysaccharides andother polymers is disclosed in U.S. Pat. Nos. 4,323,123 and 4,440,228.

Another type of polysaccharide which has been experimented with in thearea of profile control is the non-xanthan, heteropolysaccharide S-130.S-130 belongs to the group of non-xanthan welan gums. S-130 is producedby fermentation with a microorganism of the genus Alcaligenes. Anotherwelan gum heteropolysaccharide, known as S-194, is also produced byfermentation with a microorganism of the genus Alcaligenes. A notablecharacteristic of the heteropolysaccharide S-130 is that it develops ahigh viscosity in saline waters. This is particularly so in brines whichcontain divalent cations such as Ca²⁺ and Mg²⁺ or monovalent cationssuch as Na⁺ and K⁺.

U.S. Pat. No. 4,658,898 discloses the use of welan gum S-130 in salinewaters. Crosslinking with trivalent cations, such as chromium, aluminum,zirconium and iron is also disclosed. Additionally, crosslinking withorganic compounds containing at least two positively charged nitrogenatoms is disclosed in U.S. Pat. No. 4,658,898, which is herebyincorporated by reference in its entirety.

Ser. No. 940,682, now U.S. Pat. No. 4,787,451, filed December 11, 1986,a co-inventor of which is also the inventor of the present invention,discloses the use of melamine-formaldehyde and other amino resins tocrosslink various polymers including the welan gum heteropolysaccharideS-130. Ser. No. 940,682 is hereby incorporated by reference in itsentirety.

A major problem which has attended the use of organic polymers asprofile control agents is that of stability in the reservoir over time.Not only must the gel formed by the polymer be stable enough towithstand the relatively high temperatures encountered in somereservoirs - itself, a difficult requirement - but it must also bestable over a range of pH conditions so that it will have the potentialof being used in different kinds of reservoirs, e.g. sandstone,carbonate rock and others. Stability to various oilfield brines isanother highly desirable requirement. Many of the known organicgel-forming polymeric compositions are unsatisfactory in one respect oranother, e.g., temperature stability, brine stability or pH range, sothat there has been a continuing need, either for new polymers, or forways to improve the stability characteristics of gels formed from knownpolymers.

A stability related phenomenon known to affect the gel-formingcompositions having potential utility in enhanced oil recoveryapplications is that of syneresis. Syneresis is the contraction orshrinking of a gel so that liquid is exuded at the gel surface. Forexample, a gel said to exhibit 20% syneresis would take up 80% of itsoriginal volume, with the remaining 20% being expelled water. Althoughthe exact mechanism responsible for the syneresis of such gel-formingcompositions is not fully understood, it is believed to result from theover-crosslinking of the polymeric material that occurs with time. Whileit is not yet known what an acceptable level of syneresis might be forprofile control gels, it is believed that to minimize syneresis wouldenhance the effectiveness of such gels.

Accordingly, it is an object of the present invention to provide aqueouscrosslinked polymeric gel compositions having improved stability throughreduced syneresis which are useful in enhanced oil recovery operations.

It is another object of this invention to provide a stable gel whichexhibits substantially less syneresis when high temperatures areencountered.

It is a further object of this invention to provide for a substantiallystable gel of reduced syneresis which is not adversely affected by theprevailing salinity or pH level.

It is yet another object to provide for an economical, stable gel foruse as a profile control agent during enhanced oil recovery operations.

It is a yet further object of this invention to provide a method whichis effective in reducing the syneresis of aqueous crosslinked polymericgels useful in enhanced oil recovery operations.

It is still yet another object of this invention to provide a processfor selectively plugging regions of higher permeability within anoil-bearing subterranean formation to obtain improved sweep efficiencyduring a fluid flood oil recovery operation utilizing gels of improvedstability.

Other objects, aspects and the several advantages of the invention willbecome apparent to those skilled in the art upon a reading of thespecification and the claims appended thereto.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an aqueouscrosslinked gel of improved stability comprising water, a viscosifyingamount of a water-dispersible polymer, a crosslinking agent and astabilizing agent. The stabilizing agent is present in an amounteffective to reduce syneresis as compared to a like gel without saidstabilizing agent. These gels are useful in various fluid flooding oilrecovery operations, including water, steam and carbon dioxide flooding,where improved sweep efficiency is desired. Also provided is a processfor controlling the permeability of subterranean formations and a methodfor improving the stability of aqueous polymeric gels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Any water-soluble or water-dispersible polymer capable of formingaqueous gels in the presence of a crosslinking agent can be used in thepractice of this invention. Polymers of natural origin and biopolymersmay be used. A preferred class of biopolymers which may be used includethe polysaccharides produced by the action of bacteria of the genusXanthomonas on a carbohydrate. The Xanthomonas polysaccharides, theirmethod of preparation, their use in various applications in thepetroleum industry are well known and are described, for example, inU.S. Pat. Nos. 3,243,000, 3,305,016, 3,208,518, 3,810,882 and 4,413,680,to which reference is made for disclosures of these materials, theirpreparation and their use. Other polymers of natural origin that may beused include cellulose polymers, e.g., the hydroxyalkyl celluloses andcarboxyalkyl celluloses and their alkali metal and ammonium salts, asdescribed in U.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680, to whichreference is made for a detailed description of these polymers.

A particular polysaccharide which is commercially available and ispreferred for use in the present invention is the ionic polysaccharideB-1459 produced by fermentation of glucose with the bacteriumXanthomonas campestris (NRRL B-1459, U.S. Department of Agriculture).This polysaccharide is produced by culturing the bacterium Xanthomonascampestris in a well aerated medium having a pH of about 7 whichcontains commercial glucose, organic nitrogen sources, dipotassiumhydrogen phosphate and appropriate trace elements. This polymer isavailable from the Kelco Chemical Company under the trade name "Kelzan",from Pfizer under the trade name "Flocon" and from other commercialsources.

Another biopolymer which may be employed in the practice of theinvention disclosed herein is the non-xanthan welan gumheteropolysaccharide biopolymer S-130 produced by fermentation underaerobic conditions of a bacterium of the Alcaligenes species, ATCC31555. This polysaccharide is described in U.S. Pat. No. 4,342,866 towhich reference is made for a description of it and of the method bywhich it may be produced. S-130 is commercially available from the KelcoOil Field Group, a division of Merck and Co., Inc.

Any suitable polymer of acrylamide meeting the above-statedcompatibility requirements can also be used in the practice of theinvention. Thus, under proper conditions of use, such polymers caninclude various polyacrylamides and related polymers which are eitherwater soluble or water dispersible and which can be used in an aqueousmedium with the gelling agents described herein to give an aqueous gel.These can include the various substantially linear homopolymers andcopolymers of acrylamide and methacrylamide. By substantially linear ismeant that the polymers are substantially free of cross-linking betweenthe polymer chains. The polymers can have up to about 50 percent of thecarboxamide groups hydrolyzed to carboxyl groups. However, as the degreeof hydrolysis increases, the polymers often become more difficult todisperse in brines, especially hard brines. As used herein and in theclaims, unless otherwise specified, the term "hydrolyzed" includesmodified polymers wherein the carboxyl groups are in the acid form andalso such polymers wherein the carboxyl groups are in the salt form,provided said salts are water-dispersible. Such salts include theammonium salts, the alkali metal salts, and others which arewater-dispersible. Hydrolysis can be carried out in any suitablefashion, for example, by heating an aqueous solution of the polymer witha suitable amount of sodium hydroxide.

Examples of copolymers which can be used in the practice of theinvention include the water-dispersible copolymers resulting from thepolymerization of acrylamide or methacrylamide with an ethylenicallyunsaturated monomer. It is desirable that sufficient acrylamide ormethacrylamide be present in the monomer mixture to impart to theresulting copolymer the above-described water-dispersible properties.Any suitable ratio of monomers meeting this condition can be used. Underproper conditions of use, examples of suitable ethylenically unsaturatedmonomers include acrylic acid, methacrylic acid, vinylsulfonic acid,vinylbenzylsulfonic acid, vinylbenzenesulfonic acid, vinyl acetate,acrylonitrile, methyl acrylonitrile, vinyl alkyl ether, vinyl chloride,maleic anhydride, vinyl-substituted cationic quaternary ammoniumcompounds, and the like. Various methods are known in the art forpreparing said copolymers. For example, see U.S. Pat. Nos. 2,625,529,2,740,522, 2,727,557, 2,831,841, and 2,909,508. These copolymers can beused in the hydrolyzed form, as discussed above for the homopolymers.

A preferred group of copolymers useful in the practice of the presentinvention are the copolymers of acrylamide or methacrylamide and amonomer such as the well known 2-acrylamido-2-methyl propanesulfonicacid AMPS® monomer. (AMPS® is a registered trademark of the LubrizolCorporation of Cleveland, OH). Useful monomers, such as the AMPS®monomer, and methods for their preparation are described in U.S. Pat.Nos. 3,507,707 and 3,768,565, the disclosure of which is incorporated byreference. The AMPS® monomer is commercially available from the LubrizolCorporation. The alkali metal salts, such as sodium2-acrylamido-2-methylpropane sulfonate are also useful in the practiceof this invention. These are also readily available.

Copolymers of acrylamide with said AMPS® monomer, and/or its sodiumsalt, are known and useful in the practice of this invention. For anexample of such a copolymer, see the above-mentioned U.S. Pat. No.3,768,565. A number of these copolymers are available from HerculesIncorporated, Wilmington, Delaware; for example, Hercules SPX-5024, a90:10 acrylamide/AMPS® sodium salt copolymer; Hercules SPX-5022, an80:20 acrylamide/AMPS® sodium salt copolymer; Hercules SPX-5023, a 50:50acrylamide/AMPS® sodium salt copolymer; and Hercules SPX-5025, a 30:70acrylamide/AMPS® sodium salt copolymer.

Another presently preferred group of copolymers for use in the practiceof the invention are the copolymers of acrylamide or methacrylamide witha monomer such as those which are the subject of U.S. Pat. No.3,573,263, the disclosure of which is incorporated by reference in itsentirety. These useful monomers include the well known commerciallyavailable material (acryloyloxyethyl) diethylmethyl ammonium methylsulfate, commonly referred to as DEMMS and the commercially availablematerial (methacryloyloxyethyl) trimethylammonium methylsulfate alsoknown as MTMMS.

Copolymers of acrylamide with said DEMMS monomer are commerciallyavailable, for example, an 80:20 acrylamide/DEMMS copolymer. Copolymersof acrylamide with said MTMMS monomer are also commercially available,for example, Hercules Reten® 210, a 90:10 acrylamide/MTMMS copolymer;and Hercules Reten® 220, an 80:20 acrylamide/MTMMS copolymer.

Other copolymeric materials which when used to form aqueous gels canbenefit from the novel aspects of this invention are disclosed in U.S.Pat. No. 4,785,028, the contents of which are incorporated by referencein their entirety.

The polymers are generally used at concentrations ranging from 1,000 to5,000 ppm in order to achieve the desired gel consistency; in mostcases, however, concentrations of 1,000 to 3,000 ppm will be adequateand about 2,000 ppm is normally preferred, although reservoir conditionsmay require other concentrations.

Crosslinking agents useful in the practice of this invention includetransitional metal ions, phenolic resins and amino resins. Suitablecrosslinking agents include polyvalent metal cations such as Al⁺³, Cr⁺³,Fe⁺³, Sb⁺³ and Zr⁺⁴. Also suitable for crosslinking are multifunctionalamines such as diamines. For example, aluminum citrate can be admixedwith the polymer or in slugs alternating with polymer slugs. Solublecompounds of Cr⁺³ or Fe⁺³ can be used, or oxidizable compounds ofdivalent iron such as FeCl₂ can be used in conjunction with an oxidant.

In the practice of this invention, a pre-formed phenolic resin can beused; said resin generally obtained by the condensation of phenol orsubstituted phenols with an aldehyde such as formaldehyde, acetaldehydeand furfural. Additionally, the phenol and aldehyde constituents can beadded separately to produce the compositions of this invention, ratherthan being added as a pre-formed phenolic resin.

Any suitable water-dispersible phenol can be used in the practice ofthis invention. Phenolic compounds suitable for use in the presentinvention include phenol, resorcinol, catechol, 4,4'-diphenol,1,3-dihydroxynaphthalene, pyrogallol, phloroglucinol and other similarcompounds. Resorcinol and phenol are the preferred phenolics for mostwater and carbon dioxide drive applications, with resorcinal beingparticularly preferred. The choice of a phenol compound will be basedlargely on the rate of gelation desired. Mixtures of the named phenolsmay also be found to be useful.

A broad range of water-dispersible aldehydes are useful in the practiceof the present invention. It is known that both aliphatic and aromaticmonoaldehydes and dialdehydes can be used. The useful aliphaticmonoaldehydes include those containing from one to ten carbon atoms permolecule, such as formaldehyde, paraformaldehyde, acetaldehyde,proprionaldehyde, butylaldehyde, isobutylaldehyde, heptaldehyde andothers. Among the useful dialdehydes are glyoxyl, glutaraldehyde andterephthaldehyde. Mixtures of the various, aforementioned aldehydes arealso useful in the practice of the present invention. Of the preferredaldehyde compounds, formaldehyde is particularly preferred.

Amino resins may either be preformed resins, such as the preferredmelamine/formaldehyde resins, mixtures of amino compounds and aldehydecompounds or mixtures of preformed resins and aldehyde compounds. Theaforementioned aldehyde compounds are also useful in the amino resincrosslinking agents of this invention. Particularly preferred aminoresins are disclosed in Ser. No. 940,682, filed December 11, 1986, whichis incorporated by reference in its entirety.

Of the transitional metal ions useful in the practice of this invention,Cr⁺³ ions are particularly preferred for forming gels. Chromic nitrateand chromic chloride have also been utilized to form gels. The pH mayoptionally be adjusted before crosslinking. Redox systems such as sodiumdichromate and sodium bisulfite have been utilized to obtain Cr⁺³ ions.Similar redox systems are described in U.S. Pat. No. 3,749,172, which ishereby incorporated by reference. When forming these gels, Cr⁺³ ions areused in a preferred amount of from about 100-750 ppm. As is understoodby those skilled in the art, the amount of Cr⁺³ ions, or othertransition metal ions, utilized will vary depending upon the molecularweight of the particular polymer utilized. In any event, the metal ionsshould be present in an amount sufficient to obtain the desired gellingeffect.

Gels resultant from crosslinking of an acrylamide copolymer are formedin a preferred range between about pH 5 and pH 8 when forming with Cr⁺³and in a preferred range between about pH 3 and pH 10 with othercrosslinking agents. These gels can be formed in fresh water, distilledwater and synthetic sea water.

The amount of organic crosslinking agent useful in the practice of thisinvention will generally be a small but effective amount sufficient toinitiate and cause gelation of an aqueous solution of the polymericmaterial. It will generally be found that the amount of amino orphenolic resin useful to form advantageous gels will be in the range of0.02 to 5.0 weight percent. When preformed resin is not employed, theamount of the amino or phenol compound used will be in the range of 0.01to about 2.0 weight percent, with concentrations of 0.05 to 1.0 weightpercent generally preferred. The concentration of aldehyde used will bein the range of 0.01 to 3.0 weight percent, with concentrations of 0.1to 1.0 weight percent generally preferred.

It has now been discovered that certain heterocyclic compounds, whenused in the gel-forming compositions of this invention, producesignificantly more stable gels, from the standpoint of gel syneresis,than a comparable like-gel without such a compound. The heterocycliccompounds which have now been found to have a beneficial influence onsyneresis include imidazole, 1,2,4-triazole, 1,4-diaminobutane,1-vinyl-2-pyrrolidinone, pyridine, pyrrole and piperazine. Of these,imidazole, a di-nitrogen ring compound is particularly preferred. Whileimidazole has been identified as a crosslinking agent for certainpolymers useful in enhanced oil recovery in U.S. Pat. No. 4,613,631, thecontents of which are incorporated by reference in their entirety, itsuse as a syneresis-reducing stability improver for aqueous polymericgels was heretofore unknown. Additionally, although the aforementionedheterocyclic compounds are effective in reducing gel syneresis when usedin the compositions disclosed herein, other similar heterocycliccompounds have been shown to produce no such benefit. For example,triethanolamine, tris(hydroxymethyl)-aminomethane,ethylenediaminetetraacetic acid (EDTA) and others have been tested andfound to have no beneficial effect on gel stability.

The following data demonstrate the extent of the unexpected beneficialresults obtained with the polymeric gels of the present invention. Theinvention is illustrated by the following non-limiting examples:

EXAMPLE 1

This example demonstrates the effect of imidazole on reducingpolyacrylamide/phenol/formaldehyde gel syneresis. A solution of 2125 ppmunhydrolyzed polyacrylamide (obtained from Aldrich Chemical Company,Inc., of Milwaukee, WI), 1060 ppm phenol, 2000 ppm formaldehyde, and1010 ppm imidazole was prepared in a brine of synthetic sea water. ThepH was adjusted to 7.1 with 0.1N NaOH and 0.1N HCl. After standing atroom temperature overnight, the solution was sealed in two ampoules andaged at 210° F. When next observed three days later, the samples formedstiff, light yellow gels with no syneresis. These samples remainedunchanged after 4, 6 and 7 days. After 10 days the samples showed about2% syneresis.

EXAMPLE 2

This example demonstrates that the gel system of Example 1 syneresesreadily without imidazole present. A sample was prepared like Example 1,except that no imidazole was included and the pH was adjusted to 6.8.This formed a firm gel similar to those of Example 1 after 3 days. After4 days, the gel no longer adhered to the ampoule walls in that it slidwithin the ampoule when tipped. After 6 days, it showed 8% syneresis.After 7 and 10 days, the syneresis was 52 and 80%, respectively.

EXAMPLE 3

This example shows that triethanolamine, a buffer, does not stabilizethe gel of Example 2. A gel was prepared as in Example 2 with theexception that 1030 ppm of triethanolamine was added and the pH wasadjusted to 7.0. The gel so prepared aged in a subtantially similarmanner as that of Example 2 in that it exhibited 50% syneresis after 7days and 76% syneresis after 10 days.

EXAMPLE 4

This example shows that tris(hydroxymethyl)aminomethane (TRIS), also abuffer, does not stabilize the gel of Example 2. When a gel was preparedas in Example 2, with the exception that it included 1050 ppm of TRIS(pH 7.0), it aged nearly the same as the gel of Example 2 (47% syneresisafter 7 days, 72% syneresis after 10 days).

EXAMPLE 5

This example demonstrates that ethylenediaminetetraacetic acid (EDTA), adivalent cation complexer, does not stabilize the gel of Example 2. Whena gel was prepared as in Example 2 with the exception that it alsoincluded 1150 ppm of EDTA and had a pH adjusted to 7.0, it aged in asimilar manner as the gel of Example 2 (52% syneresis after 7 days, 80%syneresis after 10 days).

EXAMPLE 6

This example demonstrates that 18-Crown-6, a cation complexer, does notstabilize the gel of Example 2. A gel was prepared as described inExample 2, except that it also included 1050 ppm of 18-Crown-6 and wasadjusted to a pH of 6.7. The gel so prepared was observed to agesimilarly to the gel of Example 2 (52% syneresis after 7 days, 82%syneresis after 10 days).

EXAMPLE 7

This example shows that imidazole will stabilize a polyacrylamide/AMPS®copolymer gel for many weeks. The syneresis results of samples preparedin 6% brine (90:10 NaCl:CaCl₂) containing 5000 ppm of the Phillipscopolymer HE-E®, 1000 ppm of phenol, 1050 ppm formaldehyde and imidazolein the amounts shown below (0-1600 ppm) and aged at 210° F. were:

                  TABLE 1                                                         ______________________________________                                                        Syneresis                                                     Imidazole       (%)                                                           (ppm)           11 Weeks 19 Weeks                                             ______________________________________                                         0              90       98                                                    50             90                                                            100             80       96                                                   200             60       94                                                   400              3       10                                                   800              1        4                                                   1600             0        3                                                   ______________________________________                                    

As shown, when imidazole is present at about 400 ppm or greater, nearlycomplete protection against syneresis is provided.

EXAMPLE 8

This example shows that imidazole can delay the syneresis of anintrinsically unstable polyacrylamide gel, but does not necessarily stopit. Gel samples A and B were formed using a polyacrylamide obtained fromAldrich Chemical Company, Inc., of Milwaukee, WI, (unhydrolyzed, 5020ppm), phenol (1020 and 1080 ppm, respectively), and formaldehyde (1850and 1910 ppm, respectively) in synthetic sea water. Sample A containedimidazole (1080 ppm), while Sample B did not. The initial pH's were 7.8and 7.1, respectively. The syneresis versus duration of storage at 210°F. was:

                  TABLE 2                                                         ______________________________________                                                        Syneresis (%)                                                 Days Aged         A      B                                                    ______________________________________                                         6                0      12                                                    7                0      17                                                    8                2      35                                                   11                3      86                                                   12                4      90                                                   13                4      93                                                   18                80     97                                                   ______________________________________                                    

The pH value following the aging of Samples A and B were 6.6 and 5.7,respectively. The gel in Sample A split when syneresing to form sheetsof gel. Sample B syneresed to form a tiny plug of gel.

EXAMPLE 9

This example demonstrates that imidazole will stabilize a xanthangum-based biopolymer gel for an extended period of time. A solution of3500 ppm of a xanthan gum biopolymer (Flocon® 4800, obtained from PfizerInc., Chemicals Division, 235 E. 42nd St., New York, NY 10017), 6000 ppmof a melamine-formaldehyde resin (Parez® 613, obtained from AmericanCyanamid, Wayne, NJ), 45 ppm chromium and 104 ppm NaOH was prepared in abrine of synthetic sea water. The solution so prepared was split intotwo like samples, to which 1000 ppm of imidazole was added to onesample. The two samples were stored and maintained at 175° F. andmonitored for syneresis periodically. After 10 weeks of storage at 175°F., the gel sample containing imidazole exhibited only 5% syneresis.After 26 weeks of storage, the same gel sample exhibited only 8%syneresis. The xanthan gum-based gel sample without imidazole exhibited43% syneresis after 26 weeks. The test was voluntarily discontinuedafter 26 weeks.

EXAMPLE 10

This example compares the performance of various heterocyclic compoundsto imidazole in a polyacrylamide-based gel. A solution containing 5000ppm of an unhydrolyzed polyacrylamide (obtained from Aldrich ChemicalCo., Inc. of Milwaukee, WI), 1000 phenol and 1850 ppm formaldehyde wasprepared in a brine of synthetic sea water. The solution was split toform ten samples and additized as shown in Table 3, below. The pH ofeach sample was adjusted to about 7 using solutions of 0.1N NaOH and0.1N HCl. The samples were sealed in ampoules and aged at 210° F. Table3 presents the results of subsequent observations.

                                      TABLE 3                                     __________________________________________________________________________    COMPARISON OF EFFECT OF VARIOUS HETEROCYCLIC COMPOUNDS                        ON GEL SYNERESIS FOR POLYACRYLAMIDE GELS                                      EXAMPLE                                                                              HETEROCYCLIC     GEL SYNERESIS, IN PERCENT, AT                         NUMBER COMPOUND         1 WEEK                                                                             1.5 WEEKS                                                                            2 WEEKS                                                                             3 WEEKS                             __________________________________________________________________________    10-A   None             15   --     92    >75                                 10-B   Imidazole, 1000 ppm                                                                            0    --      2    >75                                 10-C   Piperazine, 1000 ppm                                                                           5    27     --    >75                                 10-D   1,2,4-Triazole, 1000 ppm                                                                       0    --     80    >75                                 10-E   Pyrrole, 1000 ppm                                                                              9    --     83    >75                                 10-F   Pyridine, 1000 ppm                                                                             6    --     85    >75                                 10-G   1H-Tetrazole, 1000 ppm                                                                         26   --     90    >75                                 10-H   1-Vinyl-2-Pyrrolidinone, 1000 ppm                                                              0    --     86    >75                                 10-I   1,4-Diaminobutane, 1000 ppm                                                                    0    80     --    >75                                 10-J   Hydroquinone, 1000 ppm                                                                         64   90     --    >75                                 __________________________________________________________________________

As indicated in Table 3, imidazole was found to be the most effective ofthe heterocyclic compounds. Although less effective, piperazine,1,2,4-triazole, 1-vinyl-2-pyrrolidinone, pyridine, pyrrole and1,4-diaminobutane were found to have short term syneresis-reducingproperties. None of the other heterocyclic compounds used to form thecompositions of Example 10 were found to have any syneresis-reducingbenefits, whatsoever.

Although the mechanism for syneresis reduction is not fully understood,the above examples suggest that such stabilization is not brought aboutthrough buffering or calcium complexing. It is speculated that, perhaps,these gels are stabilized by having covalently bonded "soft" cationsattached to them.

Where it is desired to obtain increased sweep efficiency, gels of thisinvention can be used to plug a previously swept portion of a formation.These gels can be directed to areas of increased porosity by utilizingany suitable method known to those skilled in the art. The permeabilitycontrol treatment may be carried out periodically, when necessary, toachieve the desired permeability profile.

One method where gels of this invention can be utilized is during awaterflooding process for the recovery of oil from a subterraneanformation. After plugging the more permeable zones of a reservoir withthe novel gels of this invention, a waterflooding process can becommenced or resumed. U.S. Pat. No. 4,479,894, issued to Chen et al.,describes one such waterflooding process. This patent is herebyincorporated by reference in its entirety.

Steamflood processes which can be utilized when employing the gelsdescribed herein are detailed in U.S. Pat. Nos. 4,489,783 and 3,918,521issued to Shu and Snavely, respectively. These patents are herebyincorporated by reference herein.

Gels described herein can also be used in conjunction with a carbondioxide flooding process, either alone, or in conjunction with acyclical steam stimulation in a heavy oil recovery process to obtaingreater sweep efficiency. Cyclic carbon dioxide steam stimulation can becommenced or resumed after plugging the more permeable zones of thereservoir with the novel gels of this invention. A suitable process isdescribed in U.S. Pat. No. 4,565,249 which issued to Pebdani et al. Thispatent is hereby incorporated by reference in its entirety. Increasedsweep efficiency can be obtained when the subject gels are used incombination with a carbon dioxide process for recovering oil. Prior tocommencement or resumption of the carbon dioxide process, the morepermeable zones are plugged with these novel gels.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be utilized without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A process for recovering oil from a subterranean oil-bearing formation having relatively high permeability zones and relatively low permeability zones penentrated by at least one injection well and at least one production well in fluid communication with a substantial portion of the formation, comprising the steps of:(a) injecting into the relatively high permeability zones of the formation via the injection well an aqueous gel-forming solution comprising water, a vicosifying amount of a water-dispersible polymer; a crosslinking agent in an amount effective to cause gelation of the aqueous solution of the water-dispersible polymer; and a stabilizing agent comprising a heterocyclic compound in an amount effective to reduce syneresis of the gel-forming composition, whereby upon gelation of the solution the zones of relatively high permeabilty of the formation are selectively plugged; (b) injecting a flooding fluid into the formation via the injection well that preferentially enters the low permeability zones; and (c) recovering fluids including oil from the formation via the production well.
 2. The process of claim 1, wherein in step (a) said heterocyclic compound is selected from the group consisting of imidazole, piperazine, 1,2,4-triazole, 1-vinyl-2-pyrrolidinone, pyridine, pyrrole and 1,4-diaminobutane.
 3. The process of claim 2, wherein in step (a) said polymer is selected from the group consisting of polyacrylamides, polysaccharides, heteropolysaccharides, cellulose ethers and mixtures thereof.
 4. The process of claim 3, wherein in step (a) said crosslinking agent is selected from the group consisting of transition metal ions, phenolic resins, amino resins and mixtures thereof.
 5. The process of claim 4, wherein in step (a) said phenolic resin comprises about 1 to 99 weight percent of at least one phenolic compound selected from the group consisting of phenol, resorcinol, catechol, phloroglucinol, pyrogallol, 4,4'-diphenol and 1,3-dihydroxynaphthaline; and about 1 to about 99 weight percent of at least one aldehyde component selected from the group consisting of aliphatic monoaldehydes, aromatic monoaldehydes, aliphatic dialdehydes and aromatic dialdehydes.
 6. The process of claim 5, wherein in step (a) said phenolic compound is phenol and said aldehyde component is formaldehyde.
 7. The process of claim 4, wherein in step (a) said amino resin is a condensate of formaldehyde and melamine.
 8. The process of claims 2, 6 or 7, wherein in step (a) said imidazole is present in an amount of greater than about 200 ppm by weight.
 9. The process of claim 8, wherein in step (a) said imidazole is present in an amount of at least about 400 ppm by weight. 